Most autoimmune diseases occur with higher frequency in women. This suggests that the autoimmune phenotype is related, at least in part, to immunologic effects from sex hormones. To date, most in vitro and in vivo studies have shown that high, pregnancy doses of estrogen and progesterone are immune suppressive. Unfortunately, these data do not explain why more women get multiple sclerosis (MS) compared to men. Evidence showing a Th1 -immune promoting effect by estrogen is minimal, although such data would help explain the gender bias in susceptibility to autoimmune disease. In the previous funding period, we presented evidence to support gender effects in the cytokine response to myelin: women with MS show dramatically skewed IFN? responses to certain myelin epitopes, whereas men with MS show an IL-5 response. In this proposal, we present evidence that: 1) endogenous sex hormone levels are associated with gender-specific cytokine responses;2) the strength of the T cell receptor stimulus, along with hormone dose, may determine whether the resulting immune response is enhanced or suppressed;and 3) low doses of estrogen promote memory CD4 T cell migration across brain endothelium. These data have lead us to hypothesize that normal ovarian cycle doses of estrogen regulate four important steps in the pathogenesis of MS: 1) promote Th1 cellular activation;2) promote lymphocyte adhesion to brain endothelium;3) regulate chemoattraction of lymphocytes to endothelium;and 4) facilitate migration of memory CD4 T cells across the blood brain barrier (BBB). This proposal will address this hypothesis by studying the migration of human peripheral blood lymphocytes across a well-characterized migration model of the BBB consisting of human brain microvascular endothelial cells. In summary, this study will define the role of estrogen in regulating: cell activation via cytokine secretion and proliferation;adhesion molecule expression and function on lymphocytes and on brain endothelial cells;attraction of lymphocytes across brain endothelium via chemokines and chemokine receptors;and invasion of the BBB via matrix metalloproteinases and migration of pathogenic lymphocytes across the BBB. These studies will provide valuable understanding of the multiple effects of estrogens and may provide molecular targets for development of future therapies as well as a better understanding of the role of sex hormones in the pathogenesis of MS.